Method and circuit for a dual supply amplifier

ABSTRACT

A circuit and method for bridging an analog signal between two integrated circuits operating at different supply voltages. The circuit is a two stage fixed gain amplifier. The first stage is a transconductance amplifier and the second stage is an operational amplifier. The first stage converts an input signal from a voltage into a current. The second stage converts the current signal to an output voltage signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. Provisional ApplicationNo. 60/223,110 filed Aug. 3, 2000, and U.S. Provisional Application No.60/224,114 filed Aug. 9, 2000 both of which are incorporated byreference herein in their entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to analog amplifiers, and in particular toa fixed gain amplifier biased from two supply voltages.

[0004] 2. Background Art

[0005] Analog integrated circuits are designed and fabricated in bipolartechnology, in MOS technology, and in technologies that combine bothtypes of devices in one process. The necessity of combining complexdigital functions on the same integrated circuit with analog functionshas resulted in increased use of digital MOS technologies for analogfunctions, particularly those functions such as analog-digitalconversion required for interfaces between analog signals and digitalsystems.

[0006] When analog devices are implemented in the same technology asdigital circuitry, the supply voltages used for the digital and theanalog devices are the same. The selection of a supply voltage for acombined analog and digital circuit forces a tradeoff of the benefits ofhigher supply voltages against the benefits of lower supply voltages. Insome cases, the semiconductor process used for a particular device maybe chosen based on the associated supply voltage.

[0007] If the supply voltage is not sufficient for proper biasing andoperation of the analog device, the analog components will induce noiseinto the analog signal. If the supply voltage is increased to properlybias the analog components, power consumption rises with the voltageincrease.

[0008] In conventional designs, the analog devices are biased with asupply voltage sufficient for proper bias and operation. In the samecircuit, the digital components are powered by a reduced voltage toreduce power consumption. The analog-digital conversion device receivingthe analog input signal is powered from the analog supply voltage.Bridging the signal from the analog supply voltage to the digital supplyvoltage occurs after the analog to digital conversion is complete.

[0009] What is needed is a circuit and method for bridging an analogsignal between two supply voltages to allow each analog device to usethe supply voltage that provides minimum signal distortion and minimumpower consumption.

BRIEF SUMMARY OF THE INVENTION

[0010] The invention comprises a circuit and method for coupling analogsignals between a circuit biased from a higher supply voltage to acircuit biased from a lower supply voltage. The invention comprises atransconductance amplifier coupled to an operational amplifier. Thetransconductance amplifier is biased using a higher supply voltage thanthe operational amplifier.

[0011] The transconductance amplifier receives an input voltage signaland converts it into an analog current. The analog current is coupled tothe operational amplifier where it is amplified and output as an analogvoltage representative of the input voltage signal.

FEATURES AND ADVANTAGES

[0012] Analog signals are bridged between devices biased with differentsupply voltages.

[0013] The higher voltage supply biases the transconductance amplifierfor a more linear response.

[0014] The higher voltage supply also increases the transconductanceamplifier signal to noise ratio.

[0015] The lower voltage supply biases the operational amplifier,reducing power consumption.

[0016] The invention is applicable to any two stage amplifier wherefirst stage is biased from a high supply voltage and the higher currentsecond (output) stage is biased from a lower voltage.

[0017] The invention allows reduced geometry devices to be implementedin the analog signal path.

BRIEF DESCRIPTION OF THE FIGURES

[0018] The present invention is described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements. Additionally, the leftmostdigit(s) of a reference number identifies the drawing in which thereference number first appears.

[0019]FIG. 1 illustrates an analog front-end circuit.

[0020]FIG. 2 illustrates a dual supply fixed gain amplifier.

[0021]FIG. 3 illustrates a circuit embodiment of a dual supply fixedgain amplifier.

[0022]FIG. 4 illustrates the steps of a method for bridging an analogsignal between two supply voltages.

DETAILED DESCRIPTION OF THE INVENTION

[0023] I. Example Circuit Application

[0024] Before describing the invention in detail, it is useful todescribe an example circuit environment for the invention. The dualsupply fixed gain amplifier invention is not limited to the analogfront-end circuit environment that is described herein, as the dualsupply fixed gain amplifier invention is applicable to other analogfront-end and non analog front-end applications as will be understood tothose skilled in the relevant arts based on the discussions givenherein.

[0025]FIG. 1 illustrates an analog front-end 100 that is used to couplean analog signal into a digital device. The analog front-end 100comprises a fixed gain amplifier 140 coupled to a programmable gainamplifier 130 and an analog-digital converter 150. A first supplyvoltage 160 is coupled to the programmable gain amplifier 130 and thefixed gain amplifier 140. A second supply voltage 170 is coupled to thefixed gain amplifier 140 and the analog-digital converter 150.

[0026] A positive analog signal 110 and a negative analog signal 120 areinput to the programmable gain amplifier 130 and amplified by a desiredgain. The positive analog signal 110 and a negative analog signal 120are the positive and negative components of a differential signal. Theamplified positive signal 115 and the amplified negative signal 125 areinput to the fixed gain amplifier 140 and amplified by a second desiredgain. The second amplified positive signal 135 and the second amplifiednegative signal 145 are input into the analog to digital converter 150and converted to a digital output 180 that is representative of thedifference between the positive analog signal 110 and the negativeanalog signal 120.

[0027] II. Dual Supply Amplifier

[0028] An embodiment of the present invention is now described. Whilespecific methods and configurations are discussed, it should beunderstood that this is done for illustration purposes only. A personskilled in the art will recognize that other configurations andprocedures may be used without departing from the spirit and scope ofthe invention.

[0029]FIG. 2 illustrates a dual source fixed gain amplifier 200. Thedual source fixed gain amplifier 200 comprises a transconductanceamplifier 210 coupled to a differential output operational amplifier240. The transconductance amplifier 210 is biased from the first supplyvoltage 160. The operational amplifier 240 is biased from the secondsupply voltage 170. In one embodiment, the second supply voltage 170 isless than the first supply voltage 160.

[0030] The amplified positive signal 115 is applied to thetransconductance amplifier 210 and converted into a positive analogcurrent 217 proportional to the amplified positive signal 115. Theamplified negative signal 125 is applied to the transconductanceamplifier 210 and converted into a negative analog current 218proportional to the amplified negative signal 125. The positive analogcurrent 217 is applied to the positive input 221 of the operationalamplifier 240. The negative analog current 218 is applied to thenegative input 222 of the operational amplifier 240. The operationalamplifier 240 acts to cause the second amplified positive signal 135 tobe the product of the positive analog current 217 and a resistor 230 a.The operational amplifier 240 acts to cause the second amplifiednegative signal 145 to be the product of the negative analog current 218and a resistor 230 b.

[0031]FIG. 3 illustrates a circuit embodiment of a dual source fixedgain amplifier 300. The transconductance amplifier 210 and theoperational amplifier 240 are shown in dotted lines. The dual sourcefixed gain amplifier comprises a transistor M1 and a transistor M2coupled between the first supply voltage 160 and a positive inputtransistor M3. A resistor R1 is coupled between M1 and the first supplyvoltage 160. A transistor M6 and a transistor M7 are coupled between thefirst supply voltage 160 and a negative input transistor M5. A resistorR2 is coupled between M7 and the first supply voltage 160. A resistor Rpis coupled between the positive input transistor M3 and a tail currenttransistor M4. A second resistor Rn is coupled between the negativeinput transistor M5 and the tail current transistor M4. Transistors M1and M7 are biased by a first bias 320. Transistors M2 and M6 are biasedby a second bias 310 and the tail current transistor M4 is biased by athird bias 330. The positive input 221 is coupled between transistors M2and M3. The negative input 222 is coupled between transistors M5 and M6.The operational amplifier 240 is biased from a second supply voltage170.

[0032] The amplified positive signal 115 is applied to M3 causing M3 toconduct a positive analog current 217 supplied from M1 and M2. Thepositive analog current 217 is proportional to the amplified positivesignal 115. The amplified negative signal 125 is applied to M5 causingM5 to conduct a negative analog current 218 from M6 and M7. The negativeanalog current 218 is proportional to the amplified negative signal 125.In summary, transistors M3 and M5 convert the differential voltageformed by 115,125 to differential currents 217,218. In one embodiment,the transconductance gain of the voltage/current conversion is 1. Thetransistors M1 and M2 operate as current sources for M3. Likewise, M6and M7 also operate as current sources for M5.

[0033] The positive analog current 217 is applied to the positive input221. The negative analog current 218 is applied to the negative input222. The operational amplifier 240 maintains the second amplifiedpositive signal 135 equal to the voltage caused by the positive analogcurrent 117 flowing through the resistor 230 a. The operationalamplifier 240 maintains second amplified negative signal 145 equal tothe voltage caused by the negative analog current 118 flowing throughthe resistor 230 b. In summary, the operational amplifier amplifies thedifferential current represented by 221,222 to generate a differentialoutput voltage having components 135,145, where amplification isdetermined by the feedback resistors 230 a and 230 b.

[0034] The first supply voltage 160 minus the voltage drop across R1,M1, M2, M3, Rp and M4 is equal to zero. Allowing a larger voltage dropacross R1, M1 and M2 improves the linear response of M1 and M2 as acurrent source. The improved linear response equates to reduced noise onthe positive analog current 217. Less noise improves the signal to noiseratio in the positive analog current. Similarly, increasing the voltagedrop across R2, M6 and M7 improves the linearity of M6 and M7 as acurrent source. The improved linear response reduces noise and increasesthe signal to noise ratio in the negative analog current 218. With thevoltage drop across M3, Rp and M4 held constant, a larger voltage dropacross R1, M1 and M2 requires a larger first supply voltage 160. Alarger value for the first supply voltage 160 will improve the signal tonoise performance in the transconductance amplifier 210.

[0035] The operational amplifier 240 maintains the positive analogcurrent 217 through the resistor 230 a. The power consumed in generatingthe second amplified positive analog signal 135 is the product of thesecond supply voltage 170 and the positive analog current 217. Powerconsumption in the operational amplifier 240 will be reduced if thesecond supply voltage 170 is reduced. For example, in the embodimentwith an input voltage of 1.25V±100 mV and the first reference voltage160=2.5V, and the second reference voltage 170=1.8V, the positive analogcurrent 217=10 mA. The power dissipated in the operational amplifier 240is reduced by the product of the supply voltage reduction and thecurrent flowing at that voltage. For this example thereduction=(2.5−1.8)(10 mA)=7 mW. In other words, the dissipated power isless than it would be if both power supplies 160 and 170 were operatedat 2.5 voltage.

[0036] The gain of the operational amplifier 240 could be varied byadjusting the resistor 230.

[0037] Reduced geometry semiconductor processes generally use reducedsupply voltages for device biasing. Reduced geometry processes are alsocapable of operating at faster clock rates because of the shortenedsignal paths and reduced propagation delay. The present invention can beused to bridge an analog signal between different semiconductor geometryprocesses and implement devices in the semiconductor geometry thatoptimizes the devices function. In the analog front-end 100, bridging ananalog signal from the first supply voltage 160 to the second supplyvoltage 170 enables the analog-digital converter 150 to be implementedin a reduced geometry process.

[0038] The present invention is also applicable to other two stageamplifiers embodiments. The supply voltage bridge can be used when afirst stage requires a high bias voltage for linear operation and anoutput stage requires a lower bias voltage to reduce power consumption.

[0039] The preferred embodiment has been described using differentialinput and output signals. Alternate embodiments include single-endedinput and output signals. The circuit modifications necessary toimplement a single-ended signal bridge between different supply voltageswould be apparent to one of skill in the art.

[0040]FIG. 4. illustrates a method of amplifying an analog signalconsistent with the invention discussed herein. In step 410, an analogvoltage is received at a first stage, the first stage biased with afirst reference voltage. In step 420, the analog voltage is transformedinto an analog current, the analog current proportional to the analogvoltage. In step 430, the analog current is coupled to a second stage,the second stage biased with a second reference voltage. In step 440,the analog current is amplified and output as a second analog voltage,the second analog voltage representative of the amplified analogcurrent.

[0041] Conclusion

[0042] While various embodiments of the present invention have beendescribed above, it should be understood that they have been presentedby way of example only, not limitation. Thus, the breadth and scope ofthe present invention should not be limited by any of the abovedescribed exemplary embodiments and arrangements, but should be definedonly in accordance with the following claims and their equivalents.

[0043] The present intention has been described above with the aid offunctional building blocks and circuit diagrams illustrating theperformance of specified functions and relationships thereof. Theboundaries of the functional building blocks and circuit diagramsillustrating the performance of specified functions and relationshipsthereof. The boundaries of the functional building blocks have beenarbitrarily defined herein for the convenience of the description.Alternate boundaries-can be defined so long as the specified functionsand relationships thereof are appropriately performed. Any suchalternate boundaries are this within the scope and spirit of the claimedinvention. One skilled in the art will recognize that these functionalbuilding blocks can be implemented using discrete circuit components,circuit components constructed on an IC chip, or any combinationthereof. Thus, the breadth and scope of the present invention should notbe limited by any of the above described exemplary embodiments, butshould be defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. An amplifier, comprising: an input; a firstanalog stage coupled to said input, and a second analog stage coupled tosaid first analog stage and an output, wherein said first analog stageis biased with a first voltage and said second analog stage is biasedwith a second voltage.
 2. The amplifier of claim 1, wherein said inputis a differential input and said output is a differential output.
 3. Theamplifier of claim 1, wherein said first analog stage is atransconductance amplifier.
 4. The amplifier of claim 1, wherein saidsecond analog stage is a differential output operational amplifier. 5.The amplifier of claim 1, wherein said first analog stage ismanufactured with a first semiconductor process and said second analogstage is manufactured using a second semiconductor process.
 6. Anapparatus, comprising: a first analog gain stage having a transcondanceamplifier that is biased with a first power supply, saidtransconductance amplifier having an input adapted to receive a voltagesignal and having an output adapted to generate a current output signal:a second analog gain stage having an input coupled to said output ofsaid first gain stage and an output, said second gain stage biased withsecond power supply voltage that is lower than said first power supplyvoltage.
 7. An system, comprising: a first amplifier adapted to receivea differential input analog signal, said first amplifier biased with afirst reference voltage; a converter outputting a digital signalrepresentative of said differential input analog signal, said converterbiased with a second reference voltage; and a second amplifier coupledbetween said first amplifier and said converter, said second amplifierbridging said differential input signal from said first referencevoltage to said second reference voltage.
 8. The system of claim 7,wherein said second amplifier further comprises: a first stage biased atsaid first reference voltage, said first stage coupled to an output ofsaid first amplifier; a second stage biased at said second referencevoltage, said second stage coupled between said first stage and saidconverter.
 9. The analog system of claim 8, wherein said first stage isa transconductance amplifier and said second stage is an operationalamplifier.
 10. A method of amplifying an analog signal comprising thesteps of: (1) receiving an analog voltage at a first stage, said firststage biased with a first reference voltage; (2) transforming saidanalog voltage into an analog current, said analog current proportionalto said analog voltage; (3) coupling said analog current to a secondstage, said second stage biased with a second reference voltage; (4)amplifying said analog current and outputting a second analog voltagerepresentative of said amplified analog current.